1. Field of the Invention
The present invention relates to an optical output level control device for an optical wavelength multiplexer included in an optical transmission system. More particularly, in an optical transmission system of the type including a plurality of multiplexers, an optical wavelength multiplexer and optical fiber transmission paths connecting them, the present invention is concerned with an optical output level control device for maintaining the optical power output of the wavelength multiplexer at a constant level.
2. Description of the Background Art
Reference will be made to FIGS. 12A and 12B for describing a specific optical transmission system including optical wavelength multiplexers each being implemented with a conventional optical output level control device. As shown in FIG. 12A, a transmitting station, generally 1, includes two multiplexers 10 (sometimes referred to as multiplexers #1 and #2) and an optical wavelength multiplexer 12 each constituting a transmitting section. As shown in FIG. 12B, a receiving station, generally 2, includes an optical wavelength multiplexer 14 and two multiplexers 16 (sometimes referred to as multiplexers #1 and #2) each constituting a receiving section.
The multiplexer #1 of the transmitting station 1 is connected to the multiplexer #1 of the receiving station 2 via an optical fiber 302, the wavelength multiplexer 12, an optical fiber 314, the wavelength multiplexer 14, and an optical fiber 324. Likewise, the multiplexer #2 of the transmitting station 1 is connected to the multiplexer #2 of the receiving station 2 via an optical fiber 304, the wavelength multiplexer 12, the optical fiber 314, the wavelength multiplexer 14, and an optical fiber 326. The wavelength multiplexer 12 is connected to the wavelength multiplexer 14 by the optical fiber 314.
At the transmitting station 1, the multiplexer #1 multiplexes three low-speed optical signals, not shown, to thereby output a high-speed optical signal having a wavelength xcex1. The xcex1 high-speed optical signal is fed from the multiplexer #1 to the wavelength multiplexer 12. Likewise, the multiplexer #2 multiplexes three low-speed optical signals, not shown, to thereby output a high-speed optical signal having a wavelength xcex2. The xcex2 high-speed optical signal is also fed from the multiplexer #2 to the wavelength multiplexer 12. The wavelength multiplexer 12 multiplexes the xcex1 and xcex2 high-speed optical signals, amplifies the resulting multiplex signal to a preselected level with an exciting optical signal whose intensity is determined by a control signal based on the number of wavelengths to be multiplexed. The number of wavelengths to be multiplexed is set by a dip switch circuit or stored in a rewritable memory and is xe2x80x9c2xe2x80x9d in this specific case. The amplified multiplex signal is sent to the wavelength multiplexer 14 of the receiving station 2 via the optical fiber 314.
At the receiving station 2, the wavelength multiplexer 14 amplifies to a preselected level the received multiplex signal attenuated by the optical fiber 314 with an exciting optical signal whose intensity is determined by a control signal representative of xe2x80x9c2xe2x80x9d the number of wavelengths to be multiplexed. This number is also set by a dip switch circuit or stored in a rewritable memory and is xe2x80x9c2xe2x80x9d in this case. The wavelength multiplexer 14 separates, or demultiplexes, the amplified multiplex signal into the xcex1 and xcex2 high-speed optical signals. The separated xcex1 and xcex2 signals are respectively input to the multiplexers #1 and #2 included in the receiving station 2. These multiplexers #1 and #2 each demultiplexes the respective input signal into the original three low-speed optical signals.
The above system configuration accords to an SDH (Synchronous Digital Hierarchy) transmission system based on a new synchronous interface as prescribed by ITU-T (Telecommunication Standardization Sector of International Telecommunication Union) Recommendations G.707 and G783. In this case, the low-speed signals each has an STM-0 (Synchronous Transfer Module Level Zero) frame structure as prescribed by TTC (Telecommunication Technology Committee) of Japan and corresponding to the above ITU-T Recommendations. The high-speed signals each has an STM-1 frame structure as also prescribed by TTC and shown in FIGS. 13A and 13B.
The wavelength multiplexer 12 at the transmitting station 1 has an optical multiplexing 102, a multiplex number setting 104, a light source control 106, and an optical amplifier 108 made up of an exciting light source 110 and an amplification 112. Likewise, the wavelength multiplexer 14 at the receiving station 2 has a multiplex number setting 104, a light source control 106, an optical amplifier 108 made up of an exciting light source 110 and a n amplification 112, and an optical demultiplexing 122. The multiplex number setting 104, light source control 106 and optical amplifier 108 included in each of the wavelength multiplexers 12 and 14 constitute a conventional optical output level control device.
The operation of the transmitting station 1 is as follows. The multiplexers #1 and #2 respectively output high-speed optical multiplex signals 302 an d 304 respectively having the wavelengths xcex1 and xcex2. The signals 302 and 304 are input to the optical multiplexing 102. The optical multiplexing 102 is implemented b y an optical combiner for combining the input signals 302 and 304 and delivering the resulting multiplex high-speed optical signal 306 to the amplification 112 which is implemented by an optical fiber type amplifying circuit. The amplification 112 combines the high-speed optical signal 306 and an exciting optical signal 312 output from the light source 110. Then, the amplification 112 amplifies only signal light contained in the combined optical signal to a preselected level and sends the amplified signal light to the amplification 112 of the receiving station 2 via the optical fiber 314.
The conventional optical output level control device will be described specifically hereinafter. The optical amplification gain of each amplification 112 can be varied by varying the amount of optical power, i.e., the intensity of an exciting signal output from the associated exciting light source 110. Each amplification 112 can amplify only a particular optical wavelength band. If the amplification 112 has a specific gain characteristic shown in FIG. 14, then it can collectively amplify a plurality of wavelengths lying in the 1.55 xcexcm wavelength band.
It sometimes occurs that an optical amplifier capable of amplifying, e.g., four wavelengths is used to deal with only two or three wavelengths, depending on the optical transmission system to which the amplifier is applied. In light of this, the conventional optical output level control device includes, in addition to the optical amplifier 108, the multiplex number setting 104 for setting the number of wavelengths to be multiplied and the light source control 106 for controlling, based on the number of wavelengths, the amount of optical power to be output from the light source 110.
Assume that the number of wavelengths should be increased, e.g., from two to three, as sometimes desired due to system extension. FIGS. 15A and 15B show a condition wherein the wavelengths xcex1 and xcex2 have their optical power level P1 amplified by the optical output level control device to a necessary optical power level P2 by A dB. In this specific case, xe2x80x9c2xe2x80x9d is set by the multiplex number setting 104 as the number of wavelengths. FIGS. 16A and 16B show a condition wherein the wavelengths xcex1 and xcex2 and an additional wavelength xcex3 have their optical power levels P1 amplified by the optical output level control device to an optical power level P3 by B dB, but the level P3 is short of the necessary level P2.
Stated another way, in the case of FIGS. 15A and 15B, optical power of P2+P2, i.e., 2P2 appears on the output 314 of the amplification 112. Even when the number of input wavelengths i s increased from two (FIGS. 15A and 15B) to three (FIGS. 16A and 16B), only the same optical power of 2P2 is available on the output 314 of the amplification 112. As a result, the output for a single wavelength is 2P2/3 which is short of the necessary power level P2. In any case, when the number of input wavelengths is increased, the necessary power level P2 is not achievable unless the optical power to be output from the light source 110 is increased.
On the other hand, assume that the number of wavelengths is reduced from three to two, as sometimes desired due to, e.g., system reduction. FIGS. 17A and 17B show a condition wherein the input power level P1 of the three input wavelengths xcex1, xcex2 and xcex3 is amplified by the optical output level control device to the necessary optical power level P2 by A dB. In this specific case, xe2x80x9c3xe2x80x9d is set by the multiplex number setting 104 as the number of wavelengths to be multiplexed. FIGS. 18A and 18B show a condition wherein the wavelength xcex3 is omitted, and the optical power levels P1 of the other wavelengths xcex1 and xcex2 is amplified by the optical output level control device to an optical power level P4 by C dB, but the level P4 is higher than the necessary level P2. It will therefore be seen that when the number of input wavelengths is reduced, the necessary power level P2 is not achievable unless the optical power to be output from the light source 110 is reduced.
The operation of the receiving station 2 shown in FIG. 12B is as follows. The amplification 112, like the amplification 112 of the transmitting station 1, amplifies the high-speed optical signal attenuated by the optical fiber 314 to a preselected level and delivers the amplified signal to the optical demultiplexing 122. The optical demultiplexing 122 is implemented by an optical splitter for separating the input signal into the high-speed signals having the wavelengths xcex1 and xcex2, respectively. The xcex1 and xcex2 signals are respectively input to the multiplexers #1 and #2. The multiplexers #1 and #2 each demultiplexes the associated high-speed signal into the original three low-speed optical signals.
The multiplex number settings 104 of the above conventional system each is implemented by a dip switch circuit or an EEPROM (Electrically Erasable Programmable Read Only Memory) or similar rewritable memory.
Further, as shown in FIGS. 8A and 8B, the wavelengths input from the multiplexers to the optical wavelength multiplexer are sometimes different in optical power from each other. In this condition, the optical power output from the wavelength multiplexer would become irregular. To solve this problem, it has been customary to provide the input side or the output side of the optical amplifier with an optical branch unit, an optical power monitor, an optical variable attenuator and an attenuator control circuit for each of different wavelengths so as to reduce a difference in optical power between the wavelengths.
However, the problem with the dip switch scheme or the EEPROM scheme is that when the number of wavelengths is varied, it is necessary to alter the setting of the dip switch circuit or to update the data stored in the EEPROM by a troublesome procedure. In addition, the alteration of the dip switch setting or the updating of the EEPROM is apt to bring about errors.
Moreover, when the wavelengths input from the multiplexers to the optical wavelength multiplexer are different in optical power from each other, the wavelength multiplexer must be provided with a particular combination of an optical branch unit, an optical power monitor, an optical variable attenuator and an attenuator control circuit for each of different wavelengths. This undesirably sophisticates the circuit arrangement.
It is therefore an object of the present invention to provide an optical output level control device for an optical wavelength multiplexer eliminating the need for the alteration of the setting of a dip switch circuit or the rewriting of data stored in a rewritable memory when the number of wavelengths is varied.
It is another object of the present invention to provide a simple optical output level control device for an optical wavelength multiplexer capable of controlling, even when wavelengths output from multiplexers are different in optical power level from each other, the wavelengths to the same optical power when output from an optical wavelength multiplexer.
In accordance with the present invention, in an optical output control device for an optical wavelength multiplexer included in an optical transmission system including at least a first and a second multiplexer and a first optical wavelength multiplexer situated at a transmitting station, the first multiplexer includes a first multiplexing circuit for transforming a plurality of preselected low-speed optical signals input thereto to corresponding electric signals, and multiplexing the electric signals to thereby output a high-speed signal A first transmitting circuit receives the high-speed signal from the first multiplexing circuit, generates first wavelength data representative of the wavelength of light output from the first multiplexer, inserts the first wavelength data at a preselected position of the high-speed signal, and transforms the high-speed signal with the first wavelength data to a first high-speed optical signal having the above wavelength. The second multiplexer includes a second multiplexing circuit for transforming a plurality of preselected low-speed optical signals input thereto to corresponding electric signals, and multiplexing the electric signals to thereby output a high-speed signal. A second transmitting circuit receives the high-speed signal from the second multiplexing circuit, generates second wavelength data representative of the wavelength of light output from the second multiplexer, inserts the second wavelength data at a preselected position of the high-speed signal, and transforms the high-speed signal with the second wavelength data to a second high-speed optical signal having the above wavelength. The first optical wavelength multiplexer includes a first wavelength multiplexing circuit for multiplexing the wavelength of the first high-speed optical signal and the wavelength of the second high-speed optical signal, and a first control light amplifying circuit. The first control light amplifying circuit receives the high-speed optical signal from the first wavelength multiplexing circuit, receives the first wavelength data from the first transmitting circuit, receives the second wavelength data from the second transmitting circuit, counts different wavelengths on the basis of the first wavelength data and second wavelength data, and amplifies the high-speed optical signal with an exciting optical signal whose intensity is determined by a control signal based on the number of wavelengths counted.